The present invention relates to feed rate control method and apparatus in numerical control system, more specifically to feed rate control method and apparatus in numerical controlled machine tool capable of performing high precision and smooth operation. Also, the present invention relates to feed rate control method and apparatus in numerical control system with minimum shock in continuous machining of small machining blocks and capable of high speed machining.
As computer technology and numerical control technique progress, significant improvements are seen in machining speed and accuracy based on numerical control using computers. Apparatus capable of performing various curved surface machining such as free curved surface, etc. are commercially available.
A numerically controlled machine tool interprets the machining data (shape data) of the workpiece to be machined by reading an NC tape or other recording media. A path of movement and feed rate of a cutting tool are set for each machining block or machining unit based on the Interpreted data.
In one example, disclosed in U.S. Pat. No. 4,894,594 or European Patent application (Publication number 299080) is an acceleration/deceleration control apparatus for automatically decelerating by reading the feed rate previously set in an NC program in advance and determining where to start deceleration so that the feed rate is always less than the specified feed rate for the machining block at the start position of the machining block. Although the deceleration operation is automatic, there remains the time consuming effort to set the feed rate in the NC program.
As described before, in the conventional numerical control system, machining modes such as the path of a cutting tool, feed rate, etc. are set based on the data read out of recording media such as an NC tape.
However, machining shapes of the workpiece to be machined are non-uniform and the time required for processing each machining block varies over a wide range. On the other hand, there requires a certain time for the numerical control system to interpret the data depending on its processing speed. This means that the data interpretation of one machining block must be completed before actual processing of the one machining block. In order to meet the requirement, it was the conventional technique to provide a buffer memory to save the interpreted data for a plurality of blocks so that the saved data is made available from the buffer memory for machining without causing time lag.
It is to be noted, however, that the memory capacity of the buffer memory is limited and is normally minimized for simplicity or cost reduction purpose. If there is no residual memory capacity of the buffer memory, there may be a case where the required interpretation data is not entered before actual machining if the processing time for a machining block is shorter than the interpretation time, i.e., if the machining portion is very short.
Illustrated in FIG. 4 is the path of the machining tool (program command path) in an example of linear machining of machining blocks N1 through N12 at a constant program command feed rate Fc. An arrow for each block represents the direction and its length represents the distance of movement of the tool. The blocks N11 and N12 show deceleration steps for machining corners.
Illustrated in FIG. 5 is the change of the program command feed rate Fc in the machining steps in FIG. 4. In this example, Fc is constant, thereby shortening the operation time as the machining becomes shorter.
Accordingly, if the program operation time for each block is shorter than the minimum time of movement Ta as illustrated in FIG. 6, a movement command pulse is not in time, thereby causing the feed rate to change in a step manner. As a result the feed rate becomes noncontinuous to cause mechanical shock in the machining tool. This will result in poor machining accuracy.
On the other hand, disclosed in Japanese patent laid-open No. 195409/90 is a feed rate control method capable of easing the load to the machine and feed motor without degrading machining efficiency in a machining program containing both small and large blocks of movement. This is a feed rate control method for a numerical control system to control the feed rate of the tool along the instructed path in accordance with a machining program. It features in comprising the steps of calculating a proper feed rate at command path corners between blocks decelerating the feed rate of the tool to the proper feed rate until the start point of the block is reached in a case where the distance of movement of the block is larger than a predetermined value, accelerating the feed rate of the tool from the start point of the block to reach the maximum feed rate, and setting the feed rate of the tool during the movement in the block within the range between the proper feed rate at the start point of the block and the proper feed rate at the end point in a case where the distance of movement of the block is smaller than a predetermined value.
There is no technical idea in this prior art to introduce the tolerable minimum movement time Ta taking the processing speed in the numerical control system into consideration. Accordingly, it contains the inconvenience as illustrated in FIG. 6.
It is therefore an object of the present invention to provide a method and apparatus for controlling feed rate in numerical control system to realize always stable and highly accurate machining regardless of the length of each machining block.
It is another object of the present invention to provide a feed rate control method in numerical control system capable of minimizing the tolerable minimum moving time and achieving high speed machining.
It is still another object of the present invention to provide a feed rate control method in numerical control system capable of avoiding mechanical shock even if a series of small machining blocks may exist and also capable of high speed machining.